This invention relates to a battery separator for use in flooded cell type lead acid batteries, and to a method for making the battery separator.
In flooded cell type lead acid batteries the positive and negative plates are separated by a battery separator. The battery separator currently used by most flooded cell type lead acid battery manufacturers is of the microporous polyolefin type. This type of separator has a composition consisting essentially of a polyolefin (preferably a high molecular weight polyolefin such as an ultra-high molecular weight polyethylene ("UHMW PE")), a filler such as an amorphous silica and a plasticizer such as a processing oil, together with certain minor ingredients such as an antioxidant, lubricant and carbon black.
Microporous polyolefin separator material is commercially manufactured by passing the ingredients through a heated extruder, passing the extrudate generated by the extruder through a die and into the nip formed by two heated calender rolls to form a continuous web, extracting a substantial amount of the processing oil from the web by use of a solvent, drying the extracted web, slitting the web into lanes of predetermined width, and winding the lanes into rolls.
Such separators and a method of manufacturing them are described in U.S. Pat. No. 3,351,495.
Microporous polyolefin battery separators typically have a configuration comprising a backweb having a predetermined thickness, and parallel ribs spaced apart a predetermined distance extending outwardly from one planar surface of the backweb. The ribs extend continuously in a longitudinal direction parallel to the edges of the separator material. The thickness of the backweb and height and spacing of the ribs is specified by the battery manufacturer to maximize desired battery characteristics. Electrolyte is contained in the space formed by the ribs, backweb and the plate contacted by the ribs, and a certain amount is contained in the pores of the separator material itself and in the active material of the plates.
Such ribs are formed during manufacture of the microporous polyethylene separator by providing that one of the two heated calender rolls is engraved with grooves so that the ribs are formed as an integral part of the web produced by the calender.
There are many different specifications required by battery manufacturers relative to backweb thickness, rib size, rib spacing and width of the rolled up separator material. In manufacturing the separator material, frequent changes of the engraved roll are required to meet different customer requirements. Such changes require that the manufacturing line be shut down. Thus, valuable manufacturing time is lost, and waste scrap is generated during start-up of the line with a new engraved roll.
It is an object of this invention to provide a new method of applying ribs to a microporous polyolefin separator that minimizes lost manufacturing time and scrap generation created by changing engraved calender rolls. It is a further object to provide a new configuration of microporous polyolefin separator material.